Scarlet fever is making a comeback — and no one knows why

Scarlet fever is reemerging in certain parts of the world for reasons unknown to researchers and health officials, reports Vox.

Scarlet fever is a bacterial infection caused by the bacteria Streptococcus pyogenes. The infection, spread via coughing or sneezing, causes a red rash on the body, high fever, and sore throat, and can lead to serious health complications such as heart and kidney damage.

While the illness became more manageable with the development of antibiotics in the mid-20th century, scarlet fever cases mysteriously fell in the 19th century without the help of those medications. Over the years, scarlet fever cases have continued to fluctuate, with many parts of the world currently seeing another reemergence of the illness.

England and Wales saw three times the amount of scarlet fever cases in 2014 compared to the year before, according to a study published Nov. 27 in Lancet Infectious Diseases. Vietnam, South Korea, Hong Kong and China have also reported significant increases in scarlet fever cases since 2009. While the U.S. does not require states to track scarlet fever, the CDC told Vox there are no signs of an uptick in national cases.

Researchers proposed several explanations for the infection's re-emergence. Changes to people's immune systems may be making them more susceptible to S. pyogenes. A different pathogen may also be co-infecting individuals, thereby predisposing them to scarlet fever, according to the report.